This invention relates generally to navigation systems and more particularly to a method and apparatus for measuring movement by detecting and counting beat frequency cycles.
Various navigation systems are well known in the art that use at least two transmitters which are separated by a predetermined distance. Both transmitters generally emit radio frequency signals that are precisely related with respect to time, phase, or frequency. A remotely located receiver, most often aboard a vehicle or vessel that requires position information relative to the transmitters, detects the signals from the transmitters after propagation delays that are proportional to the distances between the receiver and the transmitters. When the detected signals from the transmitters are compared at the receiver, a time, phase or frequency difference will be apparent that is a result of the difference between the particular propagation delays associated with the transmitters. Each propagation delay difference will form a locus of points that fall on one curve of a hyperbola with the two transmitters at the foci thereof. Such navigation systems are generally referred to as hyperbolic systems and various examples thereof are illustrated by the following U.S. Patents: Donnelley, et al U.S. Pat. No. 2,218,907; Dingley U.S. Pat. No. 2,413,694; Henry U.S. Pat. No. 2,472,158; Rich U.S. Pat. No. 2,682,049; Knight U.S. Pat. No. 3,206,751 and Honore et al U.S. Pat. No. 3,242,492.
Another known hyperbolic navigational system comprises a plurality of receivers that are positioned to receive a signal transmitted by a transmitter aboard a vehicle. The number of cycles received by each receiver from the transmitter are counted and the resulting counts are used to determine the movement of the vehicle with respect to the receivers. Such a system is disclosed by the Ulstad U.S. Pat. No. 3,995,273.
Also well known are radar systems that make use of the Doppler effect frequency shift to detect the radial velocity of a moving target with respect to a stationary radar set or to discriminate between fixed and moving targets. In either case, the frequency of the return echo from the target will be shifted according to the radial velocity of the target. This Doppler frequency shift is generally detected by combining the transmitted frequency with the return echo frequency to produce a beat frequency that is proportional to the target radial velocity. Additionally, each cycle of the resulting Doppler beat frequency will correspond to a target radial movement of one-half the wavelength of the transmitted frequency.
The above described hyperbolic navigation systems, however, require signals that must be precisely related through means that may require difficult or expensive adjustment and maintenance. Since the systems rely upon propagation delay to establish the position of the receiver, any alteration in the propagation delay caused by, for example, diurnal atmospheric changes, must be accurately anticipated and corrections must then be made when computing the resulting hyperbolic curve. With respect particularly to the Ulstad patent, the system disclosed therein does not provide navigational information aboard the vehicle and the system is apparently best suited to applications involving only one vehicle since an additional transmitter aboard an additional vehicle would require at least another set of receivers and counters. Finally, while Doppler radar systems may be used for navigational purposes, the equipment is complex and expensive; also various sources of error occur within a typical system, including, for example, compass error associated with the measurement of ground speed and drift angle, and errors introduced due to the low-level return signal received when an aircraft is operated over still water.